(1) Field of the Invention
The present invention relates to a method and an apparatus for measuring the metal exposure in a resin covering area of a metal container or a constituent member thereof. More particularly, the present invention relates to a measurement method and an apparatus in which the metal exposure in a seam, a fabricated area, a lap-seamed area or a bonded area of a metal container or a constituent member thereof such as a can body or a can lid can be detected with respect to fine parts discriminately.
(2) Description of the Prior Art
In the field of metal containers, at least the inner surface of a container is ordinarly covered with an organic resin lacquer so as to prevent dissolution of a metal into a content and corrosion of the metal. For this purpose, a metal blank coated with an organic resin is used in the production of a body or lid of a container, a stripe coating is applied in a side seam formed by welding or the like, and repair coating is applied on a scored area or riveted area of a can lid or a side-seamed area or bonded area between a lid and a body by spraying an organic resin lacquer.
As means for evalauting the metal exposure in a resin covering area in a metal container or a contituent member thereof, there is known the so-called enamel rater test. At this enamel rater test, an electrolytic aqueous solution is filled in a container or the like, a measuring electrode is immersed in this electrolytic solution, a voltage is applied between this electrode and the metal of the container, and the degree of the metal exposure is detected as a leak current between them.
At the above-mentioned enamel rater test, however, only the entire metal exposure of the container or the constituent member thereof is determined as an integrated value of the leak current, and it is difficult to evaluate the degree of the metal exposure at respective parts of the resin covering area discriminately.
The defect of the metal exposure in the resin coated area of a metal container or a constituent member thereof tends to appear at specific parts relatively to a processing tool or a processing operation. It is obvious that if the degree of the metal exposure is precisely evaluated at respective parts, the exposure of the metal can be prevented and the problem of dissolution of the metal or corrosion of the metal container can be solved by modifying a processing tool or improving a processing operation.